Is there such thing as lets say -600ma ?

Well not really. There is such a thing as -600 mA and even -600 uA.

The minus sign comes depends where your reference is for the current measurement. MA is milliamps, uA is microamps.
A should be capitalized because it's a proper name. If I really wanted to get on your case I'd say -600. MA with the decimal point to show precision.